For a number of years, it has been known that certain metals can be heat treated or otherwise processed to provide a material which has a flow stress which is dependent upon the strain rate during a forming operation. When this relationship is quite pronounced, the material has been known as superplastic metal, since it can be formed in a manner similar to molten glass or plastic. After being formed, the superplastic metal can be heat treated to produce a stronger metal part or component. As is well known in the superplastic art, such heat treatment improves the creep resistance and the tensile strength; however, in many applications the additional step in heat treatment is not required. The metal will hold its formed shape sufficiently for some uses. For background of certain features found in superplastic metal, U.S. Pat. No. 3,340,101 and U.S. Pat. No. 3,420,717 are incorporated by reference herein.
Various alloys can be transformed into a superplastic state for ease in forming. This metal can be elongated many times without necking and can flow into intricate shape when formed at an elevated temperature and at a relatively slow flow or strain rate. After forming, added strength may be obtained, if needed, by heat treating the formed metal. The most prevalent of these metals now in use is the known eutectoid alloy of zinc and aluminum. This alloy includes approximately 78% zinc and 22% aluminum. Other metals, in minor proportions, can be added to obtain certain improved or different properties. The eutectoid aluminum-zinc alloy is generally known in the trade by several trademarks and somewhat generically as Zn22Al. The present invention relates to the use of this particular material; however, other materials being capable of transformation into a superplastic state could be used in accordance with the invention. All of these metals have certain known properties used in defining superplastic metals. Also, certain soft, pliable metals such as soft copper, soft copper alloy, tin and silver can be processed in accordance with the present invention, as will become apparent in the description of the invention.
The present invention relates to the particular art of producing tooling of the compression type wherein two platens carrying die pieces are forced together to form a sheet of soft, pliable metal into the desired shape having surface impressions defined by the surface impressions of one or more of the die pieces. In this particular art, it is often desirable to cause a deep pattern when the metal being formed is relatively thin. Such applications involve simulated engraved patterns and surface contours which are found in many decorative items such as dishes now generally formed from silver plated brass and similar metals. In this art, it is common commercial practice to use a matched set of front and back dies to impress the embossed pattern and contours onto the metal being pressed between the die pieces. The front die piece is produced into the mirror image of the desired surface design. The back die piece or the die for the back side of the sheet workpiece is produced as close as possible to the final configuration of the compressed part. To produce sharp crisp detail in high relief situations, the back die or die piece must be manually fitted to the front die to achieve sufficient and uniform reproduction of the details on the sheet being compression formed. If this matching is not done accurately, the sheet may rupture during forming or it may not assume the exact detail of the embossed design of the front die piece, especially when the embossed design is deep compared to the thickness of the formed sheet. This manual production of the back die piece to be used in conjunction with a front die piece having the particular embossed design, is quite expensive and therefore adds significantly to the cost of the finished parts being formed between the two dies. This renders some applications of this particular tooling procedure uneconomical. The metal being formed in this particular type of tooling is generally a metal whose flow stress is generally independent of the strain rate, i.e. having a strain rate sensitivity below about 0.2. In other words, such metal is not superplastic. To overcome some of the difficulties experienced in making embossed items from flat metal sheets, superplastic metal can be used in its superplastic state. The parts can be heat treated, if required, by heating on the tooling or externally to provide better creep and strength properties. When a superplastic metal sheet is being employed for making embossed items, the tooling necessary for the forming operation can be reduced in cost by an amount which makes the tooling acceptable for various embossed items. In making relatively thick parts from superplastic metal sheets, while in a superplastic state, the depth of the embossing is relatively small compared to the final metal thickness of the item being formed; therefore, it is not necessary to provide matched tooling as used in forming embossed items and other metals. For instance, a machined front die part can be used with a generally flat or gradually curved rear or back die part. The superplastic metal is then compressed between the die parts or die pieces and embossing or surface impressions on one die piece is impressed upon one surface of the item being formed. This has proven successful when relatively thick items are being produced as the surface impressions are relatively well reproduced in the finished item. However, when the superplastic metal being formed is relatively thin sheet material, the use of a relatively flat or gradually contoured back die piece is not successful. Such uses distort the surface impressions, produce substantially varying thicknesses over the item being formed and form incomplete impressions of the desired surface embossing. Thus, when thin sheets of superplastic metal are being compression molded, it is still necessary to produce matching die pieces which require the hand machining previously described. Because of the finer details obtainable by superplastic metal sheet material when compressed between two die pieces, more accurate matched tooling is often needed when thin sheets of superplastic metal is being formed. Thus, the prime economical advantage of superplastic metals, which is low tooling costs, is not realized when thin superplastic metal sheet is being embossed with high resolution surface impressions by compression molding. When any soft, pliable metal in thin sheet form, such as silver, annealed copper, annealed copper alloys and tin, are being used to produce an item having detailed surface impressions or designs, it has heretofore been considered necessary to use independently matched and/or machined die pieces. This is quite expensive, involves long tooling lead time and requires highly skilled tool and die craftsmen.
The present invention relates to an improvement in tooling wherein the advantages of superplastic metal is realized even when the item being compression molded is relatively thin, without requiring matching die pieces in the tooling system.